This invention relates generally to flowmeters of the vortex-shedding type whose obstacle assembly includes a deflectable section excited into vibration by fluidic oscillations, and more particularly to a vortex meter which is substantially insensitive to acceleration forces whereby the meter provides accurate readings regardless of shock waves or other forces other than fluidic oscillations which seek to excite the deflectable section.
In many industrial processes, one must be able to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. It is also necessary, in some instances, to determine the mass flow of the fluids. Existing types of vortex flowmeters are capable of effecting volumetric flow or mass flow measurement.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic fluidic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed, these being known as Karman vortex streets. The periodicity at which vortices are shed in a Karman vortex street is a function of flow rate. In order to convert a volumetric reading to a reading of mass flow, one must multiply the volume measurement by the density of the fluid being measured.
In the Burgess Pat. No. 3,888,120, the disclosure of which is incorporated herein by reference, the obstacle assembly mounted in a flow tube through which the fluid to be metered is conducted is formed by a front section fixedly mounted across the tube and a rear section cantilevered from the front section by means of a flexible beam to define a gap serving to trap the Karman vortices. Because the rear section is deflectable, it is excited into mechanical vibration by the vortices at a rate whose frequency is proportional to fluid flow.
In the Vortex Flowmeter Model 10 LV 1000 manufactured by the Fischer and Porter Company of Warminster, Pennsylvania, the assignee of Burgess Pat. No. 3,888,120 as well as of the present application, a strain-gauge cartridge is used to sense the deflection of the rear section in relation to the fixed front section of an obstacle assembly. This strain-gauge sensor is constituted by a steel beam having a pair of high-impedance, semi-conductor strain gauges glass-bonded thereto. The characteristics of these gauges are such as to give rise to resistance changes of 0.66% for a 0.001 inch deflection at the tip of the cartridge, so that the sensor is highly sensitive and produces an electrical signal whose amplitude and frequency depends on flow rate.
In a 10 LV 100 model vortex meter and in commercially-available meters operating on similar principles, the relatively heavy deflectable section of the obstacle assembly which is suspended from a single beam has freedom of motion in two planes. The deflectable section is free to move from side-to-side with respect to its neutral position in a lateral plane, hereinafter referred to as the X plane. It can also move up-and-down with respect to its neutral position in an axial plane, hereinafter referred to as the Y plane.
When the deflectable section moves in either the X or Y plane, bending of the beam occurs. This bending action imposes a strain on the beam-mounted sensor to generate an output signal which reflects the extent of movement and the frequency or repetition rate thereof.
In normal operation, the flow of fluid past the obstacle assembly produces vortex shedding, giving rise to a fluidic force which alternates from one side of the deflectable section to the other, thereby causing this section to vibrate in the X plane at a frequency proportional to flow rate.
When, however, the vortex meter is installed in a pipeline, it may in certain cases be subject to acceleration forces. Thus in a field installation in which the meter is included in the piping of a large industrial process system which incorporates heavy machinery or explosive activity, vibratory or shock wave forces may be transmitted by the piping to the meter. These extraneous forces are picked up by the meter and cause the beam-supported deflectable section to behave in a manner comparable to the spring-mounted inertial mass of an accelerometer to produce an output signal that is a function of acceleration forces applied in the X and Y planes.
Hence in a field installation in which the vortex meter is exposed to acceleration forces, the output signal will not accurately reflect flow rate, for this signal is a composite which includes a spurious acceleration component.